This invention relates to a porous calcium silicate filler material. More particularly, the invention relates to acetylene gas storage vessels having an asbestos-free calcium silicate filler material therein and a method for manufacturing same.
Acetylene is widely used in oxy-acetylene torches because it enables temperatures of up to 3500.degree. C. to be reached for the welding and cutting of metals. However, acetylene gas is difficult to store because it is unstable and can decompose to its elements, carbon and hydrogen, with explosive violence at pressures greater than about 2 atmospheres if not properly stabilized. To provide for safe storage of acetylene gas, the gas must be dissolved in a solvent. Acetylene gas is thus typically stored in the form of an acetylene gas solution dissolved in, for example, an acetone solvent, in a vessel containing a porous filler mass. The storage vessel may be a steel cylinder. In this way, acetylene can be safely stored and shipped under pressures of up to about 15 atmospheres.
The porous filler mass comprises a capillary system of interconnecting micropores. Typically the porous filler mass is a hardened calcium silicate mass having a porosity of about 90%. The calcium silicate mass allows sufficient surface area to aid in maximum contact between the solvent and the acetylene. This system will absorb acetylene at a rate approaching 0.58 lbs. of acetylene per pound of the solvent.
The acetylene-containing cylinders are produced by filling the cylinder with the porous filler mass and injecting the solvent into the cylinder. Acetylene is then introduced into the cylinder and is distributed throughout the capillary system of the porous material as a result of its dissolution in the solvent. In this way, it is possible to insure safe storage of dissolved acetylene in quantities of up to eight times the volume of the gas which could be stored without the porous mass/solvent system.
The calcium silicate storage mass is made by mixing quicklime (calcium oxide) into water to form an aqueous slurry. Ground quartz silica is added to the slurry. A reinforcing agent is added during the mixing step to help create and hold a homogenous solution to insure a uniform mass throughout the cylinder after curing and drying. Traditionally, asbestos fibers have been used for this purpose. During mixing, one or more agents can be added to insure that the mass remains monolithic before the crystalline structure is formed during curing.
The solids are mixed in an aqueous solution for certain mixing times. The slurry is then pumped into cylinder shells completely filling them and is then cured, creating a crystalline calcium silicate mass in the cylinder. The mass is then dried to form a high porosity core, which allows the absorption of the solvent and the acetylene gas.
It is of great importance that the calcium silicate filler mass should be monolithic and should be substantially free of voids. Void spaces in the filler mass provide an available space for the formation of unacceptable volumes of acetylene gas with the attendant explosion risk. Thus, the filler mass must be formed with uniformly distributed very fine pores. During drying, the mass shrinkage must be kept controlled to less than 0.05% in any dimension but never to exceed 0.125 inches in a longitudinal direction inside the steel shell.
Previously, asbestos fibers were introduced into the aqueous slurry from which the calcium silicate filler mass was produced. The asbestos fibers functioned as a settling resistant or suspending agent to retard the settling or separation of the lime and silica from the water in the aqueous slurry composition prior to its hardening into the calcium silicate filler mass. In addition, in the hardened calcium silicate filler mass, the asbestos fibers acted as a reinforcing agent to help maintain the structural integrity of the filler mass.
However, asbestos fibers have now been found to pose health and pollution problems. Recent regulations by the Occupational Health and Safety Administration have made the use of asbestos much more difficult. OSHA regulations, particularly in 29 C.F.R. .sctn.1910 now state that an asbestos fiber level of just 0.1 fiber per cubic centimeter on an eight hour time weighted average is subject to regulation. In addition, the disposal of waste from the manufacturing phase of the calcium silicate storage mass makes it desirable to replace asbestos fibers with another more environmentally friendly product. Such constraints relating to health and safety conditions and to the handling of asbestos material have led to the consideration of other suspending and reinforcing agents in the calcium silicate filler mass.
One known substitute for asbestos fibers in the calcium silicate filler mass is an alkali resistant glass fiber. An acetylene storage vessel having a hardened asbestos free calcium silicate filler mass reinforced by glass fibers is disclosed in U.S. Pat. No. 4,349,463. While glass fibers are acceptable for this purpose, the cost of such alkali resistant glass fibers is rather high and glass fibers do not allow the filler mass to have the necessary compressibility because the glass fibers fracture even when only slightly compressed.
Therefore, others have tried to use organic suspending and reinforcing agents such as cellulose, together with mineral suspending agents. One known such calcium silicate filler mass is disclosed in U.S. Pat. No. 4,895,825. This mass includes cellulose reinforcing fibers, as well as a mineral suspending agent which can be either solid glass fibers or solids of purified clay. However, the cost for this filler material is still rather high. In addition, the mixing time of this material is long since it involves the steps of slaking quicklime with hot water to form a first mixture; adding additional water and stirring at a slow speed to form a second mixture; dispersing a cellulose reinforcing agent in the second mixture to form a third mixture; introducing with stirring into the third mixture a mixture of natural silica and either calcium silicate or amorphous ultrafine synthetic silica to form a fourth mixture and subsequently dispersing a second mineral suspending agent, which can be either glass fibers or purified clay, into the fifth mixture to form a sixth mixture. Only then is the sixth mixture transferred into the storage cylinder to be filled. Also, the step of slaking quicklime with hot water is hazardous as a volatile mixture is created.
Accordingly, it has been considered desirable to develop a new and improved calcium silicate storage mass and a method for manufacturing same, which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.